Nebula works in a different way than other digital audio tools, which could be better sometimes and worse others. Being the only software with a different approach, you will achieve things with Nebula which are not possible using other software compressors and vice versa.
Nebula processes blocks of samples because tries to be accurate on frequency and harmonic content and switching kernels too much often will lead to other unwanted effects like ringing noise or fake sounds.
How many blocks of samples? This is the PROG RATE value that the emulation preset shown in KERN page as RTE parameter. The emulation preset rate value is a fixed parameter by the library developer.

Fast emulation preset rate means that the compressor will react in a faster way. If a emulation preset is loaded with rate of a single sample you have exactly the speediness of a common software compressor, but you can't have this working correctly in Nebula unless kernels are very short and as result Nebula will add artifacts, noise and aliasing.

In order to tweak Nebula to react in a quick way you should add and change several tags in the .XML MAST page file. We also suggest you to create an alias version of Nebula for this task, for example, named Nebula3 Compressor. Remember you should create an alias of both files, the .XML file and the .DLL file in Windows or the .XML file and the .VST file in OSX.